ANODIC PROTECTION

Feasibility of anodic protection is firstly demonstrated and tested by Edeleanu in 1954

Corrosion control of metal structure by impressed anodic current.

Interface potential of the structure is increased into passive corrosion domain.

Protective film is formed on the surface

of metal structure which decrease the corrosion rate down to its passive current.

Can be applied for active-passive metals/alloys only.

Anodic protection can decrease corrosion rate substantially.

Acid concentration, M

NaCl, M Cor. Rate μm/y (Unprotected)

Cor. Rate μm/y (Protected)

0.5 10-5 360 0.64

0.5 10-3 74 1.1

0.5 10-1 81 5.1

5 10-5 49000 0.41

5 10-3 29000 1.0

5 10-1 2000 5.3

Anodic protection of 304SS exposed to an aerated H2SO4 at 300C at 0.500 vs. SCE

Metals which can be passivated and de-activated

The metals which can be passivated by oxidation and activated by reduction are those which have a higher oxide less soluble than a lower oxide and will thus each corrosion domain forms an angle.

The lower the apex of this angle in the diagram (such as titanium, chromium and tin etc.), the easier it will be to passivate the metal by oxidation and it will be difficult to reactivate the passivated metals by reduction.

Titanium and chromium can be passivated very easily and their passivation process will occur more often than not, spontaneously, even in the absence of oxidizing agent.

Experimental potential - pH diagram for chromium

Anodic polarization curve of AISI 304 SS in 0.5 M H2SO4

Anodic protection parameters :

(can be obtained from anodic polarization measurement)

Range of potential in which metal is in passivation state (protection range)

Critical current density Flade potential

Optimum potential for anodic protection is midway in the passive region

Flade potential (EF)

In which EFO : Flade potential at pH = 0

n : a constant (between 1 and 2) depends of metal composition and environment conditions

Metals having EF < equilibrium potential of hydrogen evolution reaction (HER) can be passivated by non oxidizing acid (i.e. titanium)

Increasing temperature will reduce the protection potential range and increase the critical current density and therefore anodic protection will be more difficult to be applied.

pH059,0nEE OFF

10

Parameters that should be considered for anodic protection

design (Flade potential is not included in the figure)

Influences of temperature and chloride concentration on anodic polarization curve of stainless steels

(schematic figure)

Anodic polarization curves of a mild steel in 10% sulfuric acid at 22 and 600C

For metals exposed in aggressive ions containing - environment

Interface potential of metal should be :

Eprot>Elogam>Eflade

Basically : Eflade is equal or slightly lower than Epp.

Schematic figure of potential range for anodic protection of a stainless steel which is susceptible to pitting corrosion in an

environment containing aggressive ions

Increasing of chloride ions concentration results in a significant decrease of protection potential range.

Consequently, in aggressive ions containing-environment anodic protection is applied only for metals which have relatively high protection potential and high pitting potential.

Increasing temperature leading to a decrease of Eprot

Schematic figure of anodic protection system for protecting inner surface of

storage tank

CATHODES FOR ANODIC PROTECTION Should be permanent and can be used as

current collector without any significant degradation.

Having large surface area in order to suppress cathodic overpotential.

Low cost. Platinum clad brass can be used for anodic

protection cathodes because this cathode has low overpotential and its degradation rate is very low, however it is very expensive.

Cathodes used in recent anodic protection systems

Comparison of anodic and cathodic protection :

Anodic protection

Cathodic protection

Applicability Active-passive metals only

All metals

Corrosives Weak to aggressive

Weak to moderate

Relative investment cost

High Low

Relative operation cost

Very low Mediums to high

Equipment Potentiostat + cathode/s

Sacrificial anodes or DC power supply + ICCP anode/s

Throwing power

Very high Low to high

Significant of applied current

Often a direct measure of protected corrosion rate

Complex

Does not indicate corrosion rate

Operating conditions

Can be accurately and rapidly determined by electrochemical measurement

Must usually be determined by empirical testing

Typical applications of anodic protection

Anodic protection has been applied to protect storage tanks, reactors, heat exchangers and transportation vessels for corrosive solutions.

Heat exchangers (tubes, spirals and plates types) including their anodic protection systems can be easily to purchase in the market.

i.e. AISI 316 SS HE is used to handle 96-98% sulfuric acid solution at 1100C. Anodic protection decreases corrosion rate of the stainless steel, initially from 5mm/year down to 0.025mm/year and therefore less contaminated sulfuric acid can be obtained.

DATA

Effect of chromium content on critical current density and Flade potential of iron

exposed in 10% sulfuric acid.

Effects of nickel and chromium contents on critical current density passivation potential in 1N and 10 N H2SO4 containing

0.5 N K2SO4

Requirement of critical protection current densities for several austenitic stainless steels (18-20 Cr , 8-12

Ni) exposed in different electrolytes

Protection current density : current density required to maintain passivity

Effect of sulfuric acid concentration at 240C on the corrosion rate and critical current density of stainless steel

Effect of stirring of electrolyte on the corrosion rate and requirement of current density to maintain passivity on a

stainless steel at 270C

Current density requirements for anodic protection

Anodic Protection Using a Galvanic Cathode A cylindrical tank of 304 stainless steel for

storing deaerated sulfuric acid (pH=0) is found to corrode rapidly. To provide anodic protection, a galvanic cathode of platinum will be installed. The tank has a diameter of 5 m and the depth of acid is 5 m.

a. Draw a labeled sketch of the polarization diagram for the tank and calculate the passivation potential versus SHE.

b. What is the area of platinum required to ensure stable passivity?

c. What will the corrosion potential be when the tank achieves passivity?

Data: 304 stainless steel: Ecor = -0.44 V vs SCE icor = 10-3 A/cm2

Tafel slope anodic = 0.07 V/decade icrit = 1.4 x 10-2 A/cm2 ipas = 4 x 10-7 A/cm2

H+ reduction on platinum i0 = 10-3 A/cm2

Tafel slope cathodic = 0.03 V/decade

SCE = +0.2416 V vs.SHE